1. Field of the Invention
The present invention relates to a ball bearing. More particularly, the present invention relates to a thrust ball bearing for undergoing a thrust load applied to power rollers which forms a toroidal type continuously variable transmission, or relates to a radial ball bearing for supporting various types of rotating shafts.
2. Description of the Related Art
Research is in progress to employ a continuously variable toroidal transmission which is schematically shown in FIGS. 1 and 2 for use as, for example, a transmission for automobiles, or various types of industrial machines. As disclosed in, for example, Japanese Utility Model Laid-Open No. 62-71465, a toroidal type continuously variable transmission is constructed in such a way that an input-side disk 2 is concentrically supported by an input shaft 1 and an output-side disk 4 is secured at an end of an output shaft 3. The inner surface of a casing containing the toroidal type continuously variable transmission or a supporting bracket mounted in the casing is provided with trunnions 6, 6 to be swung around axes 5, 5 located in diagonal positions with respect to the input shaft 1 and the output shaft 3.
The respective trunnions 6, 6 are provided on outer surface of both end portions with the axes 5, 5. The central portions of the trunnions 6, 6 support the base portions of respective displacement axes 7, 7. The inclinations of the displacement axes 7, 7 can be freely adjusted by swinging the respective trunnions 6, 6 around the axes 5, 5. Power rollers 8, 8 are rotatably supported around the displacement axes 7, 7 which are supported by the respective trunnions 6, 6. The power rollers 8, 8 are tightly held between the input-side disk 2 and the output-side disk 4.
Inner side surfaces 2a and 4a of the input-side and output-side disks 2 and 4 being in opposition to each other have circular arch-like shapes in section with the axes 5, 5 as the centers. Spherically formed peripheral surfaces 8a, 8a of the power rollers 8, 8 are in contact with the inner side surfaces 2a and 4a.
A loading cam-type pressing device 9 is provided between the input shaft 1 and the input-side disk 2. The input-side disk 2 is pressed elastically toward the output-side disk 4 by the pressing device 9. The pressing device 9 comprises a cam plate 10 which is rotated together with the input shaft 1 and a plurality of rollers 12, 12 (for example, four rollers) held by a retainer 1. One side surface of the cam plate 10 (left-side surface in FIGS. 1 and 2) forms a cam surface 13 having irregularities in the circumferential direction. Also, an outer side surface of the input-side disk 2 (the right-side surface in FIGS. 1 and 2) forms a cam surface 14. The plurality of rollers 12, 12 are respectively supported rotatably around axes in the radial directions with respect to the center of the input shaft
In the above-structured toroidal type continuously variable transmission, when the cam plate 10 is rotated in accordance with rotation of the input shaft 1, the plurality of rollers 12, 12 are pressed by the cam surface 13 against the cam surface 14 of the input-side disk 2. As a result, as soon as the input-side disk 2 is pressed against the power rollers 8, 8 the input-side disk 2 is rotated due to-the engagement of the cam surfaces 13, 14 and the plurality of rollers 12, 12. Then, the rotation of the input-side disk 2 is transmitted via the power rollers 8, 8 to the output-side disk 4, whereby the output shaft 3 fixed to the output-side disk 4 is rotated.
When changing the rotation speed between the input shaft 1 and the output shaft 3 and first when performing deceleration between the input shaft 1 and the output shaft 3, the trunnions 6, 6 are swung around the axes 5, 5 to incline the displacement axes 7, 7 such that the peripheral surfaces 8a, 8a of the power rollers 8, 8 are brought into contact with portions of the inner side surface 2a of the input-side disk 2 close to the center thereof and portions of the inner side surface 4a of the output-side disk 4 close to the outer periphery thereof, as shown in FIG. 1.
On the other hand, when performing acceleration, the trunnions 6, 6 are swung to incline the displacement axes 7, 7 such that the peripheral surfaces 8a, 8a of the power rollers 8, 8 are brought into contact with portions of the inner side surface 2a of the input-side disk 2 close to outer periphery thereof and portions of the inner side surface 4a of the output-side disk 4 close to the center thereof, as shown in FIG. 2. When the inclinations of the displacement axes 7, 7 are set so as to be the intermediate position of FIGS. 1 and 2, it is possible to obtain the intermediate variable speed ratio between the input shaft 1 and the output shaft 3.
FIGS. 1 and 2 merely show the basic construction of the toroidal type continuously variable transmission. However, various types of more specific constructions used as automobile transmissions are conventionally known, as disclosed in, for example, a microfilm of Japanese Utility Model Application No. 61-87523 (Japanese Utility Model Laid-Open No. 62-199557).
For the operation of the toroidal type continuously variable transmission constructed as described above, the power rollers 8, 8 are rotated at a high speed while being subjected to a thrust load from the input-side disk 2 and the output-side disk 4. In order to bear such a thrust load, thrust ball bearings 15 illustrated in FIG. 3 are arranged between the power rollers 8, 8 and the trunnions 6, 6, respectively.
A thrust ball bearing 15 (in FIG. 3) comprises: a power roller 8, which also serves a function of an inner ring, that is, a first raceway ring; a plurality of balls 16, 16; a cage 20 for retaining the balls 16, 16, which can be freely rolled; and an outer ring 17, that is, a second raceway ring, which shares a central axis .alpha. with the power roller 8. The power roller 8, the balls 16, 16 and the outer ring 17 are formed of steel used for bearings, such as bearing steel, carburized steel or the like. An inner ring raceway 18, which is a first raceway, is formed at one surface (top surface of FIG. 3) along the axis of the power roller 8, while an outer ring raceway 19, which is a second raceway, is formed on one surface (bottom surface of FIG. 3) along the axis of the outer ring 17 so as to opposedly face the inner ring raceway 18. These raceways 18 and 19 are sectionally circular, but annular as a whole. The conventional thrust ball bearing 15 as described above is constructed such that the radius of curvature R18 in a cross section of the inner ring raceway 18 is equal to the radius of curvature R.sub.19 in a cross section of the outer ring raceway 19 (i.e., The balls 16, 16 abut against the inner ring raceway 18 and the outer ring raceway 19 while being rolled.
However, there presents the following problems in the thrust ball bearing 15 built into a toroidal type continuously variable transmission for practical use so as to bear the power rollers 8. The power rollers 8 which double as an inner ring are subjected to loads from two points, that is, from a point of contact between the power rollers 8 and the input-side disk 2 and another point of contact between the power rollers 8 and the output-side disk 4, these disks 2 and 4 being opposedly arranged at the opposite ends of the diameter of the power rollers 8. However, the loads are not applied to the power rollers is when the power rollers 8 are circumferentially displaced from the above-mentioned points of contact at 90.degree., thereby failing to ensure the uniform load distribution along the overall circumference of the power rollers 8. In consequence, the power rollers 8 are subjected to a stress field to which bending stress is applied. In contrast, the outer ring 17 undergoes a substantially uniform load along the overall circumference by means of the balls 16, 16 which are equally spaced along the circumference.
It is further necessary to restrict the installment positions of the power rollers 8 so as to ensure a gear ratio of the toroidal type continuously variable transmission. The power rollers 8 are thus required to have a sufficient thickness T.sub.8. The power rollers 8 having the function of the inner ring of the thrust ball bearing 15 are further subjected to a stress field in which stress intensity factor increases. This jeopardizes the life of the power rollers 8, which is determined depending on not only a mode 2 (corresponding to rolling contact fatigue) but also a mode 1 (corresponding to bending fatigue). Among modes 1 and 2, the mode 2 which can be expressed in the form of the plane shearing is concerned with the rolling fatigue caused by flaking, which fatigue shortens the fatigue life of a typical thrust ball bearing. This adversely influences not only the thrust ball bearings for toroidal type continuously variable transmissions but also general ball bearings. On the other hand, the mode 1 in the opened form is concerned with the bending fatigue which leads to cracking in the inner ring. Such cracking gives rise to fatigue peculiar to a thrust ball bearing used for the toroidal type continuously variable transmission in which a nonuniform load is applied to the power rollers 8 having a large thickness.
Hence, in order to,ensure the durability of the thrust ball bearing for the toroidal type continuously variable transmission, consideration must be taken for the mode 1 indicative of the bending fatigue, as well as the mode 2 representing the rolling contact fatigue, which is typically considered for guaranteeing durability. With a view to preventing cracking due to the bending fatigue represented by the mode 1 and to prolonging the life of the raceway ring, the raceway ring should be formed of a material having a high degree of the fracture toughness K.sub.1c. However, a material having a high degree of the fracture toughness K.sub.1c is not necessarily effective for avoiding the rolling contact fatigue represented by the mode 2 and may sometimes fail to prolong the rolling contact fatigue life represented by the mode 2. For example, for prolonging the rolling contact fatigue life by using a carburized SCr, it is necessary to some extent to have a higher content of carbon (C%) of such a material (steel) and to carburize it to a certain degree of depth. However, an increase in the higher content of C% and in the carburizing depth of the material decreases the fracture toughness K.sub.1c. As is seen from the above contradiction, among the two types of materials A and B which are formed of the same material but different compositions and have been allowed to undergo a thermal treatment, there are some cases in which the material A is more resistant to the bending fatigue represented by the mode 1 than the material B, while the material B is more resistant to the rolling contact fatigue represented by the mode 2 than the material A. This contraction requires a careful selection of a material.
For increasing the fracture toughness K.sub.1c, the following factors (1)-(4) may be considered by way of example.
(1) Decreasing the grain size of the crystal structure of a material, PA1 (2) Distributing a microfine carbide in the crystal grains, PA1 (3) Raising the tempering temperature so as to decrease the hardness level of a material, and PA1 (4) When steel undergoes surface treatments, such as carburizing, nitriding, induction hardening, and the like, the depth of the surface to be hardened is decreased and a level of hardness of the core is decreased.
From the above factor (4), the surface hardened steel results in a higher degree of a fracture toughness than fully quenched steel. Conversely, the following factors incur a reduction in the fracture toughness of the members formed of the same material: (a) an increase in the dimensions of members, (b) a reduction in the atmospheric temperature, and (c) an increase in the loading speed.
When a material having a low degree of the fracture toughness K.sub.1c cannot be used for the power rollers 8 because of the restriction of the configuration, which the power rollers 8 constitute the thrust ball bearing 15 for the toroidal type continuously variable transmission, that is, when it is necessary to ensure the resistance to the bending fatigue represented by the mode 1, there is no choice but to use a material having a low resistance to the rolling contact fatigue represented by the mode 2 and having a high degree of the fracture toughness K.sub.1c. Thus, without making any adjustments to the material, flaking is likely to occur on the inner ring raceway 18 mounted on the power roller 8 formed of the material provided with the above-mentioned characteristics. In order to use a material having a higher degree of the fracture toughness K.sub.1c and yet be substantially free from flaking, it is necessary that the radius of the curvature R.sub.18 in a cross section of the inner ring raceway 18 be decreased (approximates one half of the external diameter of the balls 16, 16) and that the area of contact between the rolling surfaces of the balls 16, 16 and the inner ring raceway 18 be increased, thereby inhibiting a maximum contact surface pressure of such a point of contact. However, a decrease in the radius of a curvature in a cross section of the raceway surface causes an increase in the rolling resistance between the raceway surface and the rolling surfaces of the balls 16, 16.
The conventional thrust ball bearing 15 described above is constructed in such a way that the radius of the curvature R.sub.18 in a cross section of the inner ring raceway 18 is equal to the radius of the curvature R.sub.19 in a cross section of the outer ring raceway 19 (i.e., Therefore, a mere decrease in the radius of the curvatures R18 and R19 in a cross section of the respective raceways 18 and 19 results in an increase in the rolling resistance applied to the thrust ball bearing 15, thereby increasing power loss of the toroidal type continuously variable transmission having such a built-in thrust ball bearing 15. Such a problem is applied not only to the thrust ball bearing 15 but to radial ball bearings depending on the conditions Of use.